One day, a boy noticed a man selling balloons that he was blowing up by himself. Being a young child, he was excited and ran to get one. When he bought it, however, it wouldn't float up in the air. Why is that?

Hint

Answer

It wouldn't fly simply because it was mouth blown. When it floats up, it's because of the type of gas inside it, which is helium. Helium is less dense than air, so it floats. Carbon dioxide, on the other hand, is not.Hide

SO sorry. But I have a list of reason why this cant be a good teaser.
1) Mouth blown baloons DO float a little
2) who would know that unless they are in mmiddle school?
There are more, but you get my point. Good luck next time.

a few things. first, you have to give the creator some slack! the difficulty rating WAS on the label thingy. second, and this is directed to lastlyboo, i knew mouth blown balloons didnt float when i was in first grade, so do not say that only middle school students could get that. sorry if it seems offensive (which im trying not to be) but its true